Classification follows the material principally deposited. A machine that primarily works with metal falls under 848510 even if it can also handle other substrates. When principal function is genuinely ambiguous, the General Rules of Interpretation and any applicable chapter notes govern; a binding ruling from CBP is advisable for borderline equipment.

Yes — 848590 covers parts of additive manufacturing machines of all material types within this heading. However, a part that is more specifically described elsewhere in the tariff schedule (for example, a laser unit or a precision linear guide) may be classified under its own heading rather than 848590.